Consolidated Statements of Cash Flows (Parentheticals) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for real estate additions	$ 3,191	$ 631	$ 705
Vornado
Liability for real estate additions	$ 18	$ 125	$ 21

Non-cash additions to real estate included in accounts payable and accrued expenses.

No definition available.

Name:	alx_NonCashAdditionsToRealEstate
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration